As per Marvel writer C.B. Cebulski's tweet last week, the Marvel Universe is indeed crossing over with Hajime Isayama's titanic manga series Shingeki no Kyojin/Attack on Titan!
Not a joke, folks. Attack on Titan and the @Marvel universe are crossing over! In 2 weeks. In Japan. Want a teaser? pic.twitter.com/9AF5fqWLfX
— C.B. Cebulski (@CBCebulski) November 3, 2014
Sources: Twitter (@CBCebulski), io9.com